1. Field of Invention
The present invention relates to a control circuit, and more particularly, to a frequency generator apparatus, which controls, using an electric fuse (efuse), whether or not a frequency generator is activated, and the control circuit thereof.
2. Description of Related Art
With the diversification of operation frequency for various electronic products and chips, the programmable frequency generator has been arranged in the chip or the electronic product by many manufacturers by design, so as to meet the requirements of operation frequency for various electronic products or chips developed by each client. For example, a frequency generator apparatus disclosed in U.S. Pat. publication No. 6,720,834 determines the output frequency of the voltage controlled oscillator by using a laser fuse.
FIG. 1 shows a frequency generator apparatus of U.S. Pat. publication No. 6,720,834. The conventional frequency generator apparatus is arranged in a chip. Referring to FIG. 1, control circuits 102, 103, and 104 have laser fuses 105, 106, and 107 respectively. The control circuit 102 has a diode 108, the control circuit 103 has two diodes 109 and 110, and the control circuit 104 has three diodes 111, 112, and 113. An output frequency FO generated by this conventional frequency generator apparatus is controlled by the voltage difference between two ends of the diode strings in the control circuits 102, 103, and 104, such that the number of the diodes decides the value of the output frequency FO. In other words, each of the control circuits represents one option of the output frequency FO. Thus, if the control circuit 102 is selected to determine the output frequency FO, the laser fuse 106 in the control circuit 103 and the laser fuse 107 in the control circuit 104 must be burnt out, such that an open circuit occurs in the control circuits 103 and 104. Similarly, if the control circuit 103 is selected to determine the output frequency FO, the laser fuse 105 in the control circuit 102 and the laser fuse 107 in the control circuit 104 must be burnt out respectively. If the control circuit 104 is selected to determine the output frequency FO, the laser fuse 105 in the control circuit 102 and the laser fuse 106 in the control circuit 103 must be burnt out.
Since a laser beam is required to change the state of the laser fuse, an opening that is large enough for the laser beam to irradiate the fuse must be retained on the surface of the chip. Furthermore, generally, in order to prevent impurities spilled when burning out the laser fuse from staining the circuits and elements around the laser fuse, effective protective measures must be taken around the laser fuse. Further, an additional laser-supplying device is required in the chip with the conventional frequency generator apparatus, which adds additional disturbance in using the conventional frequency generator.
Compared with the laser fuse, an electric fuse (efuse) does not need a laser beam to decide its state, thus, the efuse does not occupy too much area of the chip. There are two kinds of states for the efuse, that is, program state and non-program state, which present two different resistances. The construction, the program state and non-program state of the efuse are illustrated below with reference to FIGS. 2, 3-A, and 3-B.
FIG. 2 is a top view of an electric fuse (efuse) element, FIG. 3-A is a side view of the efuse element in a non-program state, and FIG. 3-B is a side view of the efuse element in a program state. Please refer to FIGS. 2, 3-A, and 3-B according to the following illustration. 201, 202 shown in FIG. 2 indicate contacts, 203 indicates a fuse region, and 204, 205 indicate contact regions. 301 and 302 in FIGS. 3-A and 3-B indicate the contacts 201 and 202 in FIG. 2. A silicide layer, a polysilicon layer, an oxide layer, and a substrate are shown in both FIGS. 3-A and 3-B, and the numeral 300 in FIGS. 3-A and 3-B indicates the efuse element shown in FIG. 2. 303-A in FIG. 3-A and 303-B in FIG. 3-B indicate the fuse region in FIG. 2.
When the efuse element 300 is at the non-program state, the fuse region therein has not burnt out yet and thereby being at the state of 303-A in FIG. 3-A. Therefore, the contacts 301 and 302 may still be electrically conducted by the silicide layer and the polysilicon layer, such that the resistance of the efuse element 300 at this time is relatively low (generally about 5Ω). However, when the efuse element 300 is at the program state, the fuse region therein is burnt out due to a large current between the contacts 301 and 302, and thereby being at the state of 303-B in FIG. 3-B. Therefore, when the efuse element 300 is at the program state, the contacts 301 and 302 are only electrically conducted by the polysilicon layer, such that the resistance of the efuse element 300 at this time is relatively high (generally about 300Ω).
However, different from the great difference between the resistance of the laser fuse in the program state and that in the non-program state (ideally, the resistance of the laser fuse in the non-program state is considered to be 0Ω, whereas that in the program state is considered to be infinity), the resistance of the efuse in the program state and that in the non-program state are very close, such that the laser fuse cannot be directly replaced by the efuse in the conventional art. Therefore, the efuse cannot be used for providing a program function in the conventional technology for controlling an oscillator.